A hydatidiform mole is a rare mass or growth that forms inside the uterus at the beginning of a pregnancy. It is a type of gestational trophoblastic disease (GTD).

See also:

• Gestational trophoblastic disease
• Choriocarcinoma (a cancerous form of GTD)

A hydatidiform mole, or molar pregnancy, results from over-production of the tissue that is supposed to develop into the placenta. The placenta normally feeds a fetus during pregnancy. In this condition, the tissues develop into an abnormal growth, called a mass.

There are two types:

• Partial molar pregnancy
• Complete molar pregnancy

A partial molar pregnancy means there is an abnormal placenta and some fetal development.

In a complete molar pregnancy, there is an abnormal placenta but no fetus.

Both forms are due to problems during fertilization. Potential causes may include defects in the egg, problems within the uterus, or a diet low in protein, animal fat, and vitamin A. Women under age 16 or older than 40 have a higher risk for this condition. You also are more likely to have a molar pregnancy if you have had one in the past.